Soft Starting Method and System Thereof in the Way of Wave-Skipping with Stepped Frequency and Stepless Voltage Regulating for a Motor

ABSTRACT

The invention relates to a soft starting method and system thereof in the way of wave-skipping with stepped frequency and stepless voltage regulating for a motor which can be applied to the large torque starting of AC motor under the condition of a power supply of a industrial frequency supply source and the safe starting of higher load. Trigger signals generated by a control system, in the soft starting method of the invention, act on five sets of anti-parallel thyristor valves connected between the power supply and the motor to conduct a pair of thyristors thereof according to a set frequency and sequence, and the motor is started from a standstill status to full speed in the way of wave-skipping by controlling the sets of the thyristor valves. The method may improve the starting torque for more than 10 times of the traditional motor soft starting of the voltage reduction and control the starting current for about two times of the rated current. The cost thereof is only 20-25% of similar frequency converter.

FIELD OF THE INVENTION

The invention relates to the 3-Step soft starting method with Large-Torque for the AC Motor which can be applied to the large torque starting of AC motor under the condition of the power supply with standard industrial frequency and the safe starting AC motor under condition of higher load.

BACKGROUND OF THE INVENTION

The solid-state starting device of AC motors, in prior art, adopts a starting method through anti-parallel thyristor bidirectional valves which can decrease the voltage applied to the motor. Due to the square of the voltage is directly proportional to the torque of the motor, the starting torque is very small but the impact current is very strong, which can be controlled 4-6 times of rated current of the motor usually, result in something abnormal such as block up or unsuccessful starting easily happened. However, the investment expense of the equipment is very high by using conventional adjustable-speed device.

SUMMARY OF THE INVENTION

To solve the technical problem in prior art, the purposes of the present invention is to provide the soft starting method and system thereof in the way of wave-skipping with stepped frequency and stepless voltage regulating for the motor which can improve the starting torque for more than 10 times of the traditional motor soft starting through voltage reduction and control the starting current for about two times of the rated current. And the cost thereof is only 20-25% of similar frequency converter.

In order to realize the purposes above-mentioned, the technical program of the invention is as follows:

The soft starting method in the way of wave-skipping with stepped frequency and stepless voltage regulating for the motor: trigger signals generated by a control system act on sets of anti-parallel thyristor valves connected between the power supply and the motor to conduct a pair of thyristors thereof according to a set frequency and a set sequence, and the motor is started from a standstill status to full speed in the way of wave-skipping by controlling the sets of the thyristor valves to realize the maximum torque of the motor and make the motor complete the starting from the static to full speed.

The said frequency and sequence can be set: three half-wave in every waveform of 4 half-wave are selectively removed, 3 of 4 half-waves in every waveform of the supply source are selectively removed, by means of ½f trigger signal, result in the fundamental wave frequency of the voltage and current of the motor being ½ of the power frequency, that is 25 Hz, to make the rotation speed of the motor increasing in full speed from ¼ to realize the starting of the motor from 12.5 Hz to 25 Hz; all of the half-wave will be applied to the motor through the trigger signal of the power frequency result in the rotation speed thereof increasing in full speed from ½ times to realize the starting from 25 Hz-50 Hz; A method of half-wave removed selection is: a cluster of 6 trigger signals from the control system is divided into 3 groups respectively connecting the sets of valve of an actuator, wherein each set of valve are of two signals at same time and the starting points of the signals from the adjoining two groups are at intervals of ⅔ times rated cycle, conduction will be made in accordance with the following sequences: the 1^(st) set of valve is positive conduction and the 5^(th) set of valve is opposite conduction simultaneously; the 4^(th) set of valve is positive conduction and the 1^(st) set of valve is opposite conduction simultaneously; the 5^(th) set of valve is positive conduction and the 4^(th) set of valve is opposite conduction simultaneously and makes the fundamental wave frequency of AC voltage at the output end being ½ of the rated frequency.

The said frequency and sequence can also be set with three steps as follow:

Step 1: while starting, 7 of the 8 half-waves in every waveform of the supply source will be selectively removed by means of the ¼f trigger signal result in the fundamental wave of the voltage of the motor being ¼ of the power frequency i.e. 12.5 Hz and the voltage steadily increasing to ¼ rated voltage, this brings about the motor starting from 0 Hz to 12.5 Hz;

Step 2: 3 of 4 half-waves in every waveform of the supply source are selectively removed, by means of ½f trigger signal, result in the fundamental wave frequency of the voltage and current of the motor being ½ of the power frequency, that is 25 Hz, to make the rotation speed of the motor increasing in full speed from ¼ to realize the starting of the motor from 12.5 Hz to 25 Hz;

Step 3: all of the half-wave will be applied to the motor, through the trigger signal of the power frequency, result in the rotation speed thereof increasing in full speed from ½ times to realize the starting from 25 Hz-50 Hz;

The method of half-wave removed selection is with the following two steps:

Step 1: the cluster of 6 trigger signals from the control system is divided into 3 groups respectively connecting the sets of valve of the actuator, wherein each set of valve has two signals at same time and the starting points of the signals from the adjoining two groups are at intervals of 4/3 times rated cycle, the conduction will be made in accordance with the following sequences: the 1^(st) set of valve SA is positive conduction and the 3^(rd) set of valve is opposite conduction simultaneously; the 2^(nd) set of valve is positive conduction and the 1^(st) set of valve is opposite conduction simultaneously; the 3^(rd) set of valve is positive conduction and the 2^(nd) set of valve is opposite conduction simultaneously and makes the fundamental wave frequency of AC voltage at the output end be ¼ of the rated frequency.

Step 2: the cluster of 6 trigger signals from the control system is divided into 3 groups respectively connecting the sets of valve of the actuator, wherein each set of valve has two signals at same time and the starting points of the signals from the adjoining two groups are at intervals of ⅔ times rated cycle, conduction will be made in accordance with the following sequences: the 1^(st) set of valve is positive conduction and the 5^(th) set of valve is opposite conduction simultaneously; the 4^(th) set of valve is positive conduction and the 1^(st) set of valve is opposite conduction simultaneously; the 5^(th) set of valve is positive conduction and the 4^(th) set of valve is opposite conduction simultaneously and makes the fundamental wave frequency of AC voltage at the output end be ½ of the rated frequency.

The system used for the soft starting method in the way of wave-skipping with stepped frequency and stepless voltage regulating for the motor includes two parts of the actuator and a control device, wherein the said actuator is a structure with sets of thyristor valves, each two thyristors from the sets of thyristor valves forms a group connecting between the power supply and the motor in a way of anti-parallel.

The number of the said thyristor groups in the sets of valve is of 3 or 5. If the number of the thyristor groups is 3, concretely, connecting structure forms: the 1^(st) set of valve is connected between the power network and the motor through the 1^(st) input end and the 1^(st) output end; the 2^(nd) set of valve is connected between the power network and the motor through the 2^(nd) input end and the 2^(nd) output end; the 3rd set of valve is connected between the power network and the motor through the 3^(rd) input end and the 3^(rd) output end; If the number of the thyristor groups in the sets of valve is 5, concretely, connecting structure forms: the 1^(st) set of valve is connected between the power network and the motor through the 1^(st) input end and the 1^(st) output end; the 2^(nd) set of valve is connected between the power network and the motor through the 2^(nd) input end and the 2^(nd) output end; the 3^(rd) set of valve is connected between the power network and the motor through the 3^(rd) input end and the 3^(rd) output end; the 4^(th) set of valve is connected between the 3^(rd) input end and the 2^(nd) output end and the 5^(th) set of valve is connected between the 2^(nd) input end and the 3^(rd) output end.

The said control device consists of a micro-controller, a phase-shifting circuit and a programmable logical circuit. The phase-shifting circuit receives the synchronizing signals and its output end is linked to the micro-controller by a zero-crossing detection circuit to transmit the synchronizing signals after 90 degree phase-shifting. The output end of the micro-controller forwards the controlled trigger signals to the gate of the thyristors mounted in the frequency conversion starting device through the programmable logical circuit respectively; the micro-controller also links to a display circuit, a command input, a communication interface and a current regulating circuit.

Compared with traditional technology the soft starting method and its system in the way of wave-skipping with stepped frequency and stepless voltage regulating for the motor in this invention has following novelty and inventiveness:

1. The starting can realize stepless regulation for the output voltage by applying property of the thyristor. Output voltage frequency can be altered by using the soft starting method of the present invention. Magnetic flux of the motor can reach approximate the rated value at points of ¼f, ½f and if by means of stepless changing the input voltage amplitude and stepped changing the input frequency of voltage applied to the motor. By doing so, the starting torque of the motor closes to the rated torque at the said 3 points which is over 10 times of the starting torque of the traditional high-voltage soft starting and the starting current can be controlled at about 2 times of the rated current in the whole starting period as well.)

The method may improve the starting torque for more than 10 times of the traditional motor soft starting through the voltage reduction, up to 80-90% of the rated torque at ¼f and ½f points, and control the starting current for about two times of the rated current. The cost thereof is only 20-25% of similar frequency converter (obtained through sets of valves in this invention).

2. The starting mode is flexible and different starting frequency combinations can be selected in accordance with the on-site loading and power network conditions. The stepped frequency conversion mode can realize various starting modes such as voltage gradient starting and current-limit starting etc. under different starting frequencies.

3. The starting of the motor can be flexibly controlled. There are three kinds of modes to stop the motor i.e. free stop, soft stop and braking stop.

4. Many communication interfaces may be realized by adopting a digital control mode (using the control device for example) for easily networking.

5. This method can also be used at the sites where the stepped speed regulating operation with requirement of ¼ rated rotation speed and ½ rated rotation speed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic circuit of a main circuit.

FIG. 2 is a schematic circuit of the control system.

FIG. 3 is a schematic circuit of Example 2.

DESCRIPTION OF THE INVENTION IN DETAIL Example 1

The method of the present invention is: Trigger signals generated by a control system act on five sets of anti-parallel thyristor valves connected between the power supply and the motor to conduct a pair of thyristors thereof according to a set frequency and a set sequence, and the motor is started from a standstill status to full speed with three stages stepped frequency in the way of wave-skipping by controlling the sets of the thyristor valves to realize the maximum torque of the motor and make the motor complete the starting from the static to full speed.

The said frequency and sequence are as follows:

When starting, 7 of the 8 half-waves in every waveform of the supply source will be selectively removed by means of the ¼f trigger signal result in the fundamental wave of the voltage of the motor being ¼ of the power frequency i.e. 12.5 Hz and the voltage steadily increasing to ¼ rated voltage, this brings about the motor starting from 0 Hz to 12.5 Hz;

When the rotation speed of the motor reaches roughly ¼ full speed, 3 of 4 half-waves in every waveform of the supply source are selectively removed, by means of ½f trigger signal, result in the fundamental wave frequency of the voltage and current of the motor being ½ of the power frequency, that is 25 Hz, to make the rotation speed of the motor increasing in full speed from ¼ to realize the starting of the motor from 12.5 Hz to 25 Hz;

All of the half-wave will be applied to the motor, through the trigger signal of the power frequency, result in the rotation speed thereof increasing in full speed from ½ times to realize the starting from 25 Hz-50 Hz.

The method of half-wave removed selection is that the cluster of 6 trigger signals from the control system is divided into 3 groups respectively connecting the sets of valve of the actuator, wherein each set of valve has two signals at same time and the starting points of the signals from the adjoining two groups are at intervals of 4/3 times rated cycle, the conduction will be made in accordance with the following sequences: the 1^(st) set of valve SA is positive conduction and the 3^(rd) set of valve SC is opposite conduction simultaneously; the 2^(nd) set of valve SB is positive conduction and the 1^(st) set of valve SA is opposite conduction simultaneously; the 3^(rd) set of valve SC is positive conduction and the 2^(nd) set of valve SB is opposite conduction simultaneously and makes the fundamental wave frequency of AC voltage at the output end be ¼ of the rated frequency.

The cluster of 6 trigger signals from the control system is divided into 3 groups respectively connecting the sets of valve of the actuator, wherein each set of valve has two signals at same time and the starting points of the signals from the adjoining two groups are at intervals of ⅔ times rated cycle, conduction will be made in accordance with the following sequences: the 1^(st) set of valve SA is positive conduction and the 5^(th) set of valve SE is opposite conduction simultaneously; the 4^(th) set of valve SD is positive conduction and the 1^(st) set of valve SA is opposite conduction simultaneously; the 5^(th) set of valve SE is positive conduction and the 4^(th) set of valve SD is opposite conduction simultaneously and makes the fundamental wave frequency of AC voltage at the output end being ½ of the rated frequency.

The system used for the soft starting method of the present invention includes two parts of the actuator and the control device, referring to FIG. 1, wherein the said actuator is the structure with five sets of anti-parallel thyristor valves. Three input ends A, B, and C of the power supply supplied by the high voltage power network connect the five sets of anti-parallel thyristor valves through a circuit breaker QF. The five sets of anti-parallel thyristor valves are positioned between the input ends A, B and C and the output ends A′, B′ and C′ (namely, connection is made to the power supply network through the input ends A, B and C and to the motor ends through the output ends of A′, B′ and C′). The 1^(st) set of valve SA is connected between the power network and the motor through the 1^(st) input end A and the 1^(st) output end A′; the 2^(nd) set of valve SB is connected between the power network and the motor through the 2^(nd) input end B and the 2^(nd) output end B′; the 3^(rd) set of valve SC is connected between the power network and the motor through the 3^(rd) input end C and the 3^(rd) output end C′; the 4^(th) set of valve SD is connected between the 3^(rd) input end C and the 2^(nd) output end B′ and the 5^(th) set of valve SE is connected between the 2^(nd) input end B and the 3^(rd) output end C′.

Referring to FIG. 2, The said control device consists of the micro-controller, the phase-shifting circuit and the programmable logical circuit. The phase-shifting circuit receives the synchronizing signals and the output end is linked to the micro-controller (by adopting TN89C196KC in the present example) to transmit the synchronizing signals (after 90 degree phase-shifting). The output end of the micro-controller forwards the controlled trigger signals to the gate of the thyristors mounted in the frequency conversion starting device through the programmable logical circuit (by adopting EPM7160S in the present example) respectively. The micro-controller is also linked to the display circuit, the command input, the communications interface and the current regulating circuit, among which:

LCD display circuit in prior art is selected as the said display circuit in the present invention. The said command input is to control data (for example, the setting of the current and the selection of the starting curve etc.) via a keyboard; RS232 or RS42485 is selected as the said communications interface linking to a computer or a control terminal; the said current regulating circuit receives feedback signals of the motor by using a current transformer, and then provides error value compared with setting value of the keyboard as control signals for the sets of anti-parallel thyristor valves; the said phase-shifting circuit, adopting a standard circuit, communicates with the micro-controller by means of the zero-crossing detection circuit. Concretely, the said zero-crossing detection circuit is linked to phases B and C of the power supply through the phase-shifting circuit and two input leads. The output signals from the zero-crossing detection circuit are sent to an input channel of the micro-controller. An integrator is set in the phase-shifting circuit which has the output voltage advanced about 90 degree compared with the ends B and C of a line to line AC voltage. During the integrating processing, irregular wave, “concavity shape”, in ordinary sine-wave, is removed from AC voltage waveform acting on the zero-crossing detection circuit. The output signals of the zero-crossing detection circuit would be changed status between high level (“1”) and low level (“0”) while an instant value of the input voltage is beyond a zero point. Therefore, a single wave synchronizing signal SYNC corresponds to the component of the fundamental wave of VAN (i.e. AC voltage of line of phase A to neutral) in frequency and phase, It means that the single wave synchronizing signal (SYNC) acting on the micro-controller synchronizes with the power supply voltage.

There are 3 stages for the motor to complete starting from standstill status to full speed:

The first stage: the control device brings about ¼f trigger signals (starting signal) to act on the five sets of anti-parallel thyristor valves connected between the power supply and the motor. 7 of the 8 half-waves in every waveform of the supply source is selectively removed by means of the ¼f trigger signal result in the fundamental wave of the voltage of the motor being ¼ of the power frequency i.e. 12.5 Hz, so that the voltage steadily increasing to ¼ rated voltage. By doing so, it will not only reduce either the starting current of the motor, or the impact to the power network, but also maintain as much as possible the maximum torque of the motor by means of delaying the starting angle for the sets of thyristor valves. And thus, this brings about the motor starting from 0 Hz to 12.5 Hz;

The second stage: in case that the rotation speed of the motor is almost up to ¼ times of full speed, the control system brings about ½f trigger signal in time. 3 of the 4 half-waves in every waveform of the supply source are then selectively removed, by means of ½f trigger signal, result in the fundamental wave frequency of the voltage and current of the motor being ½ of the power frequency, that is 25 Hz. Thus, the starting current at the moment is comparatively low and the impact to the power network is not much. And the maximum torque of the motor is maintained as much as possible by means of regulating the starting angle of the sets of thyristor valves. The starting of the motor from 12.5 Hz to 25 Hz can be realized by increasing the rotation speed of the motor from ¼ in full speed.

The third stage: When the rotation speed of the motor is basically up to ½ times of full speed, the control system brings about the trigger signal of the full frequency i.e. power frequency. In this moment, no any half-wave of the waveforms will be removed and all of the half-wave will be applied on to the motor. The rotation speed of the motor begins to increase from ½ times in full speed. And this realizes the starting of the motor from 25 Hz to 50 Hz. After a period of time, the rotation speed can be up to or close to full speed.

By doing so, the motor has completed the procedure of acceleration from static to full speed through the three stages of acceleration from 0 Hz to 12.5 Hz, 12.5 Hz to 25 Hz and 25 Hz to 50 Hz. At the completion of the acceleration, all the thyristor groups are closed and the sets of thyristor vales then short-circuits by a by-pass contactor and then the trigger pulse of the sets of thyristor valves are off. So far, a disturbance-free switchover from mode of starting to mode of operation is finished result in the motor getting into the status of rated rotation-speed operation.

In addition, the starting device provides three modes of stopping for the motor.

The 1^(st) mode is of free shopping. In this mode, the control device shuts off all sets of valves to obtain the free stopping in the status of no power. In this mode, the motor often requires a very long period of time to stop in the condition of larger inertia.

The 2^(nd) is of soft stopping. In this mode, the control device acts on the motor with the frequencies from 50 Hz, 25 Hz, 12.5 Hz to 0 Hz, in accordance with the sequence opposite to the starting procedure. This mode requires obviously shorter time than the 1^(st) mode does.

The 3^(rd) mode is of electrical-brake stopping. In this mode, the control device brings about DC component to the voltage waveform applied to the motor and controls the torque and the braking current by means of controlling the conduction angle of the sets of valves. A quick stopping can be realized by this mode.

In general, the technology in this invention can bring up the starting torque to over 10 times than the traditional motor soft starting of the voltage-reducing and the starting current is held at about 2 times of the rated current. And the cost is only 20-25% of the frequency converter.

Example 2

The soft starting method in the way of wave-skipping with stepped frequency and stepless voltage regulating for a motor can be applied to a circumstance which requires only two stages of the stepped starting (¼f and full frequency) to meet the demand of starting. In FIG. 1, the sets of thyristor valves SD and SE can be removed and the sets of thyristor valves SA, SB and SC are remained.

Referring to FIG. 3, the connecting relation of the circuits is: three input ends of power supply A, B and C are energized by the high-voltage power network and the three sets of anti-parallel thyristor valves are connected between the input ends A, B and C and the output ends A′, B′ and C′. The 1^(st) set of valve SA is connected between the 1^(st) input end A and the 1^(st) output end A′; the 2^(nd) set of valve SB is connected between the 2^(nd) input end B and the 2^(nd) output end B′; the 3^(rd) set of valve SC is connected between the power network and the motor through the 3^(rd) input end C and the 3^(rd) output end C′.

The method of removed selection is as follows. The cluster of 6 trigger signals from the control system is divided into 3 groups respectively connecting the sets of valve of the actuator, wherein each set of valve has two signals at same time and the starting points of the signals from the adjoining two groups are at intervals of ⅔ times rated cycle, conduction will be made in accordance with the following sequences: the 1^(st) set of valve SA is positive conduction and the 5^(th) set of valve SE is opposite conduction simultaneously; the 4^(th) set of valve SD is positive conduction and the 1^(st) set of valve SA is opposite conduction simultaneously; the 5^(th) set of valve SE is positive conduction and the 4^(th) set of valve SD is opposite conduction simultaneously and makes the fundamental wave frequency of AC voltage at the output end be ½ of the rated frequency. 

1. A soft starting method in the way of wave-skipping with stepped frequency and stepless voltage regulating for a motor, characterized in that trigger signals generated by a control system act on five sets of anti-parallel thyristor valves connected between a power supply and the motor to conduct a pair of thyristors thereof according to a set frequency and a set sequence, and the motor is started from a standstill status to full speed in the way of wave-skipping by controlling the sets of the thyristor valves to realize the maximum torque of the motor and make the motor complete the starting from the static to full speed.
 2. The soft starting method in the way of wave-skipping with stepped frequency and stepless voltage regulating for the motor according to claim 1, characterized in that the said frequency and sequence can be set as follows: firstly, 3 of 4 half-waves in every waveform of the supply source are selectively removed, by means of ½f trigger signal, result in the fundamental wave frequency of the voltage and current of the motor being ½ of the power frequency, that is 25 Hz, to make the rotation speed of the motor increasing in full speed from ¼ to realize the starting of the motor from 12.5 Hz to 25 Hz; secondly, all of the half-wave of the supply source applied to the motor, through the trigger signal of the power frequency, result in the rotation speed thereof increasing in full speed from ½ times to realize the starting from 25 Hz-50 Hz.
 3. The soft starting method in the way of wave-skipping with stepped frequency and stepless voltage regulating for the motor according to claim 2, characterized in that wherein a method of half-wave removed selection is as follows: a cluster of 6 trigger signals from the control system is divided into 3 groups respectively connecting the sets of valve of an actuator, wherein each set of valve has two signals at same time and the starting points of the signals from the adjoining two groups are at intervals of ⅔ times rated cycle, conduction will be made in accordance with the following sequences: the 1^(st) set of valve(SA) is positive conduction and the 5^(th) set of valve(SE) is opposite conduction simultaneously; the 4^(th) set of valve(SD) is positive conduction and the 1^(st) set of valve(SA) is opposite conduction simultaneously; the 5^(th) set of valve(SE) is positive conduction and the 4^(th) set of valve(SD) is opposite conduction simultaneously and makes the fundamental wave frequency of AC voltage at the output end being ½ of the rated frequency.
 4. The soft starting method in the way of wave-skipping with stepped frequency and stepless voltage regulating for the motor according to claim 1, characterized in that the said frequency and sequence are set with three steps as follow: firstly, while starting, 7 of the 8 half-waves in every waveform of the supply source will be selectively removed by means of the ¼f trigger signal result in the fundamental wave of the voltage of the motor being ¼ of the power frequency i.e. 12.5 Hz and the voltage steadily increasing to ¼ rated voltage, this brings about the motor starting from 0 Hz to 12.5 Hz; secondly, 3 of 4 half-waves in every waveform of the supply source are selectively removed, by means of ½f trigger signal, result in the fundamental wave frequency of the voltage and current of the motor being ½ of the power frequency, that is 25 Hz, to make the rotation speed of the motor increasing in full speed from ¼ to realize the starting of the motor from 12.5 Hz to 25 Hz; thirdly, all of the half-wave of the supply source will be applied to the motor, through the trigger signal of the power frequency, result in the rotation speed thereof increasing in full speed from ½ times to realize the starting from 25 Hz-50 Hz;
 5. The soft starting method in the way of wave-skipping with stepped frequency and stepless voltage regulating for the motor according to claim 4, characterized in that the method of half-wave removed selection is with the following two steps: step 1: the cluster of 6 trigger signals from the control system is divided into 3 groups respectively connecting the sets of valve of the actuator, wherein each set of valve has two signals at same time and the starting points of the signals from the adjoining two groups are at intervals of 4/3 times rated cycle, the conduction will be made in accordance with the following sequences: the 1^(st) set of valve (SA) is positive conduction and the 3^(rd) set of valve (SC) is opposite conduction simultaneously; the 2^(nd) set of valve(SB) is positive conduction and the 1^(st) set of valve(SA) is opposite conduction simultaneously; the 3^(rd) set of valve (SC) is positive conduction and the 2^(nd) set of valve(SB) is opposite conduction simultaneously and makes the fundamental wave frequency of AC voltage at the output end be ¼ of the rated frequency. step 2: the cluster of 6 trigger signals from the control system is divided into 3 groups respectively connecting the sets of valve of the actuator, wherein each set of valve has two signals at same time and the starting points of the signals from the adjoining two groups are at intervals of ⅔ times rated cycle, conduction will be made in accordance with the following sequences: the 1^(st) set of valve(SA) is positive conduction and the 5^(th) set of valve(SE) is opposite conduction simultaneously; the 4^(th) set of valve(SD) is positive conduction and the 1^(st) set of valve(SA) is opposite conduction simultaneously; the 5^(th) set of valve(SE) is positive conduction and the 4^(th) set of valve(SD) is opposite conduction simultaneously and makes the fundamental wave frequency of AC voltage at the output end be ½ of the rated frequency.
 6. A system used for the soft starting method in the way of wave-skipping with stepped frequency and stepless voltage regulating for the motor according to claim 1, characterized in that of including two parts of the actuator and a control device, wherein the said actuator is a structure with sets of thyristor valves, each two thyristors from the sets of thyristor valves forms a group connecting between the power supply and the motor in a way of anti-parallel.
 7. The system used for the soft starting method in the way of wave-skipping with stepped frequency and stepless voltage regulating for the motor according to claim 6, characterized in that the number of the said thyristor groups in the sets of valve is of 3 or
 5. 8. The system used for the soft starting method in the way of wave-skipping with stepped frequency and stepless voltage regulating for the motor according to claim 7, characterized in that when the number of the thyristor groups is 3, concretely, connecting structure forms: the 1^(st) set of valve(SA) is connected between the power network and the motor through the 1^(st) input end(A) and the 1^(st) output end(A′); the 2^(nd) set of valve(SB) is connected between the power network and the motor through the 2^(nd) input end(B) and the 2^(nd) output end(B′); the 3^(rd) set of valve(SC) is connected between the power network and the motor through the 3^(rd) input end(C) and the 3^(rd) output end(C′).
 9. The system used for the soft starting method in the way of wave-skipping with stepped frequency and stepless voltage regulating for the motor according to claim 7, characterized in that when the number of the thyristor groups in the sets of valve is 5, concretely, connecting structure forms: the 1^(st) set of valve(SA) is connected between the power network and the motor through the 1^(st) input end(A) and the 1^(st) output end(A′); the 2^(nd) set of valve(SB) is connected between the power network and the motor through the 2^(nd) input end(B) and the 2^(nd) output end(B′); the 3^(rd) set of valve(SC) is connected between the power network and the motor through the 3^(rd) input end(C) and the 3^(rd) output end(C′); the 4^(th) set of valve(SD) is connected between the 3^(rd) input end(C) and the 2^(nd) output end(B′) and the 5^(th) set of valve(SE) is connected between the 2^(nd) input end(B) and the 3^(rd) output end(C′).
 10. The system used for the soft starting method in the way of wave-skipping with stepped frequency and stepless voltage regulating for the motor according to claim 7, characterized in that the said control device consists of a micro-controller, a phase-shifting circuit and a programmable logical circuit; wherein the phase-shifting circuit receives the synchronizing signals and its output end is linked to the micro-controller by a zero-crossing detection circuit to transmit the synchronizing signals after 90 degree phase-shifting. The output end of the micro-controller forwards the controlled trigger signals to the gate of the thyristors mounted in the frequency conversion starting device through the programmable logical circuit respectively; the micro-controller also links to a display circuit, a command input, a communication interface and a current regulating circuit. 